Embodiments of the present invention are directed to an input voltage buffer for providing an input voltage to circuit components such as an analog-digital converter (ADC). Typically, an emitter follower-type of circuit is used as the input voltage buffer, and the output voltage at the emitter is sampled. When used in differential AC-coupled applications, it may be necessary to provide an input common-mode bias.
Often, the input bias is applied through a common-mode reference buffer, as shown in the input buffer 100 of FIG. 1. In FIG. 1, the common-mode reference, VCM 110 is applied to the input buffer 100 through a common-mode buffer 120 and two equal resistors RCM 130.1, 130.2. If it is desired to set the output bias voltage Vout of the input buffer equal to VCM, the common-mode reference buffer must level shift the reference. As shown in FIG. 1, this level shift compensates for the base-emitter voltage (VBE) of the input device, such as transistors 150.1, 150.2 and the voltage drop due to base current flow, which is equal to IAVG, across the common-mode resistors RCM 130.1, 130.2. This level shift can be created accurately based on knowledge of the input device, the input device's base current, and the common-mode resistor RCM. A circuit can be made that accurately creates a level shift with a replica (or properly scaled replica) of the input devices 150.1, 150.2 and the common-mode input resistance RCM 130.1, 130.2.
In a sampled data system, such as an ADC, there is a phase of the sample clock (hold phase) during which the input is not applied to the sampling network. In an effort to save power, the input signal buffer may be switched off during this phase. The input buffer is re-activated in the sample phase. The input buffer must recover sufficiently in the sample phase to avoid error in the sampled voltage.
Considering the input common-mode bias structure of FIG. 1, switching by switches 160.1, 160.2 of the input buffer can, however, cause a common-mode bias error even when allowing a sufficient recovery time for the signal buffer. The voltage VCM 110 is a common mode voltage that maintains a stable voltage value. A common-mode buffer 120 provides a buffered and level-shifted reference voltage. The base current of the switched input buffer device is filtered by the relatively large AC-coupling capacitors 140.1, 140.2 to produce a current approximately equal to the average of the switched current, Iavg. The flow of this average current across RCM 130 produces a possible error that must be cancelled with an appropriate level shift.
The true average of the current IB cannot easily be determined, and ringing, undershoot and overshoot cause errors in the estimated value of Iavg. Attempts have been made to approximate the average current Iavg by scaling a base current to a replica to account for the averaging. However, the switched base current IB does not have a perfect 50% duty cycle square wave but has some ringing, overshoot, undershoot, and/or other imperfections associated with the transitions. Accordingly, it is difficult to replicate the average current Iavg exactly to correct for the above described imperfections to prevent the error in common-mode bias voltage.
In more detail, the voltage at the base of transistors 150.1, 150.2 should be VCM+VBE; however, due to the unknown average of the current IB over the common mode resistance RCM 130.1, 130.2, the voltage at the base of transistor 150.1, 150.2 is Vcm+Vbe−ΔIavgRcm. Here, ΔIavg, which is the difference between the actual Iavg and the estimate of Iavg (Iavg(est)), is being used to generate the output voltage of the common-mode buffer, which is equal to Vcm+Vbe+Iavg(est)Rcm. In which case, the voltage IavgRcm is not accurately known, thereby causing error in the common mode. The current sources 170.1, 170.2 represent a combined current IB+IC.
Previous attempts to replicate an estimate of the unknown current fail to accurately solve the problem of base-current-created common-mode voltage error with a switched input buffer. Since the detrimental effects of the unknown current would not easily be negated by replicating an estimate of the unknown current, a different solution was needed.
The inventor recognized that if the common-mode resistors could be negated, replicating the unknown current would not be necessary. One way to accomplish this is to generate a “negative” impedance. Placing an appropriately valued negative impedance in series with the output of the common-mode buffer would effectively cancel the common-mode resistors. This effect could be achieved with a common-mode buffer with negative output impedance. Negative impedance converters are known. However, the widely known prior art negative impedance converter is typically used as a shunt load impedance, not as a voltage buffer. It would be beneficial to have a negative impedance converter configured for use as a voltage buffer.